Nuclear reactor operation can be described in terms of "neutron economy." In simple terms, fission of a fuel nucleus releases one or more neutrons, and one neutron is required to sustain the chain reaction. Many of the design features of a nuclear reactor are based upon their impact on the neutron economy. In particular, materials for use in reactors are selected for their neutron capture cross-sections, .sigma., along with other properties. Low .sigma. materials are selected for most reactor components, such as support structure, fuel rod cladding, moderators, and the like. High .sigma. materials are selected for particular uses such as control rods and burnable poison shims. A burnable poison shim is a high .sigma. material added in carefully selected quantities to decrease neutron flux early in a fuel cycle, and to become transparent or neutral after neutron absorption so that late in a fuel cycle more of the fission neutrons are absorbed by fuel.
The element with the highest .sigma. for the natural isotope mixture is gadolinium. However, its use is limited because as the gadolinium concentration increases, the fuel thermal conductivity and melting temperature decrease. Additionally, production of high weight percent (about 8 weight percent) gadolinium fuel is judged to be more difficult than low weight percent (less than about 4 weight percent). Only two isotopes have high absorbance, as illustrated in the following table:
______________________________________ Properties of Naturally Occurring Gadolinium Isotopes Isotope Natural .sigma. (Thermal) Mass Abundance Barns ______________________________________ 152 0.20% 125 154 2.15 102 155 14.73 61,000 156 20.47 1,000 157 15.68 254,000 158 24.87 3.5 160 21.90 0.77 ______________________________________
As illustrated, Gd.sup.155 and Gd.sup.157 have the highest .sigma. of the gadolinium isotopes but make up only about 30 percent of the natural gadolinium. The cost-to-benefit ratio of the use of gadolinium as a burnable poison shim can be decreased by using an isotope enrichment process to increase the abundance of the high .sigma. isotopes, while at the same time decreasing the negative affects.
Various processes have been proposed for the separation or enrichment of isotopes, such as those using photochemical reactions, laser techniques and ion exchange separation. A number of isotope separation processes have been based on chemical exchange, as described in Isotope Separation, S. Villani, American Nuclear Society, 1976. One method is ion exchange chromatography, in which a solution containing the element of interest, in ionic form, is passed through a column of ion exchange resins. Exchange of ions is between resin-bound and solution phase forms. As a general rule, ions containing a heavy isotope are concentrated in the heavier component, which is the resin in the case of ion exchange.
Commercial purification of rare earth elements was originally based on ion exchange separations. Of particular interest is a pair of publications by F. Molnar, A. Horvath and V. A. Khalin in the Journal of Chromatography, Vol. 26 (1976), the first entitled "Anion Exchange Behavior of Light Rare Earths in Aqueous Methanol Solutions Containing Neutral Nitrates, I. Separation of Carrier-Free Light Rare Earths" (pp. 215-224); and "Anion Exchange Behavior of Light Rare Earths in Aqueous Methanol Solutions Containing Neutral Nitrates, II. Macro-Micro Separations" (pp. 225-231). These publications report on separation of rare earth elements, and particularly on separation of gadolinium from its neighboring elements on the periodic table, europium and terbium. The chromatographic peaks shown therein have abnormal shapes, and suggest, to the present inventors, that the peak shape is being influenced by a small degree of isotope separation, although the authors did not make such an interpretation.
An object of the present invention is to provide a process that will separate desired isotopes of gadolinium from a mixture containing other gadolinium isotopes.
It is another object of the present invention to provide a process for the separation of desired isotopes of gadolinium from a mixture containing other gadolinium isotopes and other rare earth elements.
It is a further object of the present invention to provide an efficient and economical process for separating gadolinium isotopes using an ion exchange resin, with recovery of eluant solution and various by-products.